Crankshaft

ABSTRACT

A crankshaft is made of steel wherein straightening can be carried out with ease even though normalizing after hot forging is eliminated, and wherein cutting work can be carried out with ease even though lead (Pb) content of the steel is low. The steel is to be treated with ferritic nitrocarburizing after being machined to a crankshaft, and contains following chemical compositions: C: not less than 0.35 mass % (mass percentage) and not more than 0.45 mass %, Si: not less than 0.1 mass % and not more than 0.4 mass %, Mn: not less than 0.4 mass % and not more than 0.7 mass %, S: not less than 0.04 mass % and not more than 0.07 mass %, Ca: not less than 0.0005 mass % and not more than 0.0050 mass %, Ti: not less than 0.0050 mass % and not more than 0.0120 mass %, N: not less than 0.0042 mass % and not more than 0.0480 mass %, and balanced Fe and inevitable impurities, wherein Ti/N (the ratio of mass percentages of Ti and N) is controlled in the range of not less than 0.25 to not more than 1.2, and Pb is regulated to not more than 0.03 mass %. In addition, the cross-sectional microstructure except the ferritic nitrocarburized layer consists of ferrite+pearlite. In this ferrite+pearlite microstructure, the total number of sulfides, the size of which exceed 1 μm (micro-meters) observed per 160 mm 2  of the viewing field, is counted to be not less than 12,000 and not more than 31,000, and the averaged grain size of pearlite lies in the range of not less than 14 μm to not more than 20 μm.

RELATED APPLICATIONS

This application claims the priority of Japanese Patent Application No. 2004-081385 filed on Mar. 19, 2004.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to a crankshaft, more particularly, to a crankshaft made of non-leaded, non heat refined, ferritic nitrocarburized steel having an excellent bending-straightening property.

2. Description of the Related Art

Since a crankshaft for an automobile is used under such circumstances as to undergo large and repeating loads of torsion and bending, it is required to have superior static strength as well as fatigue strength. In addition to the above, since the crankshaft is a member with a considerably large and complicated shape, it is typically made of non heat refined steel which does not need any heat treatment such as quenching and tempering. Further, the crankshaft is required to be applied with surface hardening treatment in a later step of the process to increase the strength. In the case of the non heat refined steel mentioned above, the referenced patent documents Japanese Patent Laid-Open Publication No. H10-030632, H06-128690, H05-279795 and H05-279794 have disclosed ferritic nitrocarburizing treatment with respect to such surface hardening treatment. The ferritic nitrocarburizing treatment is carried out at temperatures below A1 transformation point, typically at around 570° C., in an atmosphere of ammonia gas for example. During this treatment, atoms of nitrogen as well as part of carbon penetrate into the steel surface and cause the surface hardness to increase through creating nitrides and carbo-nitrides. The ferritic nitrocarburizing treatment mentioned above neither causes a distortion to a work piece to be treated nor requires long time for treatment, therefore, is suitable for mass-production of a crankshaft that is a part of a large engine for an automobile.

With regard to the crankshaft treated with the ferritic nitrocarburizing, a straightening process is inevitably carried out after the ferritic nitrocarburizing treatment in order to remove distortion generated during hot forging or the ferritic nitrocarburizing process. In the prior art, normalizing treatment has been applied after hot forging for not only controlling the grain size of the steel microstructure but also removing the distortion so as to keep a satisfactory bending-straightening property. This normalizing treatment, on the contrary, causes to increase both the man-hour and the production cost.

In addition, the crankshaft is, generally, of such complicated shape that cutting work after hot forging is inevitable. In the prior art, the crankshaft is found to have some problems in the cutting process. That is to say, cutting chips generated during the cutting process after hot forging tangle around the work piece and consequently shorten tool life. Therefore, steels containing lead (Pb), which is known to improve the chip breakability effectively, have been widely used for the crankshaft production. However, in the recent years, Pb have been given an aversion and the usage thereof have been decreasing in terms of globally rising concern about the environment protection,

In light of the problems described above, the object of the present invention is to provide a crankshaft made of steel that enables the straightening subsequent to the ferritic nitrocarburizing to be done with ease even though normalizing subsequent to hot forging is eliminated, and also that enables cutting work to be done with ease in spite of a low Pb content.

SUMMARY OF THE INVENTION

For the purpose of achieving the foregoing and the other objects, the crankshaft according to the present invention is made of steel, which is intended to be treated through a ferritic nitrocarburizing on the surface thereof, having such compositions that:

-   C (carbon): not less than 0.38 mass % (mass percentage) and not more     than 0.42 mass %, -   Si (silicon): not less than 0.15 mass % and not more than 0.35 mass     %, -   Mn (manganese): not less than 0.45 mass % and not more than 0.6 mass     %, -   S (sulfur): not less than 0.04 mass % and not more than 0.06 mass %, -   Ca (calcium): not less than 0.0010 mass % and not more than 0.0050     mass %, -   Ti (titanium): not less than 0.0050 mass % and not more than 0.0120     mass %, -   N (nitrogen): not less than 0.0042 mass % and not more than 0.0480     mass %, -   and balanced Fe (iron) and inevitable impurities; wherein Ti/N (the     ratio of the mass percentages of Ti and N) exists in the range of     not less than 0.25 to not more than 1.2; wherein the cross-sectional     microstructure of the steel except the ferric carbonitrided layer     consists of ferrite+pearlite and the total number of sulfides, the     size of which is not less than 1 μm (micro-meters), per 160 mm²     (square milli-meters) of the viewing field in the objective     ferrite+pearlite microstructure is not less than 12,000 and not more     than 31,000; and wherein the averaged grain size of pearlite     consisting the microstructure is not less than 14 μm and not more     than 20 μm.

The crankshaft may be configured as such that crank arms, which are disposed with a predetermined spacing along a rotational axis of the crankshaft, are interconnected by means of crank journals, a center axis of which is positioned in coincidence with the above mentioned rotational axis, and crank pins, the center axis of which is positioned at a predetermined distance away radially from the above mentioned rotational axis.

Further, “the number of sulfides” according to the present invention is determined as will be described below. To begin with, a viewing field of 160 mm² (square millimeters) is chosen in an arbitrary manner in a mirror-polished cross section lying in parallel with the forging direction of the steel, which cross section is called as a “longitudinal cross section” hereinafter. Then, the viewing field is image-scanned by means of a CCD (Charge Coupled Device) image sensor with a magnification of 400, and an image of a phase having a different color from that of the matrix phase is extracted by means of an image analyzing device for calculating the area size of the respective extracted phases. Then, assuming the extracted phase as a circle, the diameter of the circle having the same area size as the extracted phase is calculated, and the number of extracted phases having the diameter larger than 1 μm is counted. With regard to the metallurgical type of the extracted phase having the different color mentioned above existing in the steel with the chemical compositions according to the present invention, more than 99% of the extracted phases have been found as sulfides, most of cations of which come from Mn (manganese), according to the preliminary EPMA (Electron Probe Microanalyzer) analysis. Therefore, the counted number of extracted phases described above can be adopted as “the number of sulfides”.

Still further, “the averaged grain size of pearlite” in the ferrite+pearlite microstructure is determined as will be described below. To begin with, the microstructure in a longitudinal cross section at a position within 3 mm from the surface layer (except the ferritic carbonitrided layer) of a crank pin of an as-forged crankshaft is observed with an optical microscope. Then, the dimensions of the pearlite grains in the viewing field are measured with respect to the two reference lines intersecting at right angles, and the average is taken from these two measurements. The measurement on this particular pearlite grain is carried out continuingly by varying the direction of the reference lines for searching the maximum averaged dimension. And this maximum averaged dimension is determined as the size of this particular pearlite grain. The sizes of all of the pearlite grains in the 30 viewing fields, which are chosen randomly and the area size of which is 0.5 mm² (square millimeters) respectively, are measured, and these sizes are averaged to determine “the averaged grain size of pearlite”.

The steel having the chemical compositions described above assumes, after hot forging and subsequent air-cooling, to have ferrite+pearlite microstructure, and to meet the aforementioned numerical requirements regarding the number of sulfides and the averaged pearlite grain size (the averaged pearlite grain size does not change after the ferritic nitrocarburizing). And by virtue of the generation of the preferable microstructure aforementioned, the straightening after the ferritic nitrocarburizing can be performed with ease even though normalizing subsequent to hot forging is eliminated. Further, the steel exhibits an excellent machinability in spite of a low Pb content. As a consequence, the crankshaft with ferritic nitrocarburizing treatment can be produced in low cost according to the present invention.

Now, the reasons for specifying the steel compositions and the numerical parameters according to the present invention will be described below.

C: Not Less than 0.38 Mass % (Mass Percentage) and not More than 0.42 Mass %.

Carbon is an essential alloying element to keep the strength. When less than 0.38 mass %, carbon is not enough to keep the strength of the steel. On the other hand, when more than 0.42 mass %, carbon causes the steel to increase the hardness so as to make the machinability worse.

Si: Not Less than 0.15 Mass % and not More than 0.35 Mass %.

Silicon is added as a deoxidizer in a steel melting process, and effective in increasing the fatigue strength. When less than 0.15 mass %, silicon does not produce the desired effects mentioned above. On the other hand, when added more than 0.35 mass %, silicon causes the ferrite phase to increase the hardness so as to make the straightening property worse.

-   Mn: Not Less than 0.45 Mass % and not More than 0.6 Mass %.

Manganese is an essential alloying element for forming manganese-type sulfides that contribute to enhance the machinability. When less than 0.45 mass %, manganese is not enough to form the required amount of the manganese-type sulfides so as to make the machinability insufficient. On the other hand, when more than 0.6 mass %, manganese causes the steel to increase the hardness so as to make the machinability worse.

-   S: Not Less than 0.04 Mass % and not More than 0.06 Mass %.

Sulfur is, as well as manganese, an essential alloying element for forming manganese-type sulfides so as to contribute to improve machinability. When less than 0.04 mass %, sulfur does not form the enough amount of the sulfides so as to make the machinability insufficient. On the other hand, when more than 0.06 mass %, sulfur causes the steel to lose toughness and ductility so as to make the steel sensitive to hot cracking during forging process.

Ca: Not Less than 0.0010 Mass % and not More than 0.0050 Mass %.

As will be described after, the content of lead (Pb), which has been added preferably for enhancing the machinability in the prior art, is reduced in the steel according to the present invention. Specifically, the Pb content of the invented steel is controlled not to exceed 0.03 mass %, which is a level of an inevitable impurity. Instead, calcium is added in order to compensate for the reduction in the machinability. Not less than 0.0010 mass % of calcium is necessary to exhibit the significant effect on the machinability. On the other hand, when more than 0.0050 mass %, the excess calcium forms a great amount of calcium sulfides (CaS) having a high melting temperature. This results in a serious obstacle arising in the casting process of molten steel. Besides calcium, bismuth (Bi) or tellurium (Te) is known to improve the machinability. However, calcium is more effective than the others in view point of enhancing the machinability because part of calcium is solid solute into manganese sulfides (MnS) so as to prevent MnS to deform in the process of hot forging.

Ti: Not Less than 0.0050 Mass % and not More than 0.0120 Mass %.

Titanium (Ti) is an alloying element combining with oxygen (0) in the steel to form finely dispersed oxides. Since such titanium oxides serve as the nucleus for the precipitation of the manganese-type sulfides, they effectively help the manganese-type sulfide to disperse finely. Further, titanium tends to combine as well with carbon and nitrogen in steels to form fine nitrides or carbo-nitrides. These nitrides or carbo-nitrides serve to prevent the coarsening of austenite grains during hot forging of crankshaft. In addition to this, these nitrides and carbo-nitrides enhance the ferrite generation in the process of ferrite-pearlite transformation during cooling from the hot forging temperature so as to result in making the pearlite grains fine. When less than 0.0050 mass %, titanium does not exhibit the positive effects described above. On the other hand, when more than 0.0120 mass %, titanium forms coarse titanium nitrides. Such coarse titanium nitrides act as the origins of stress concentration and result in the reduction of fatigue strength.

Ti/N (The Ratio of the Mass Percentages of Ti and N): not Less than 0.25 and not More Than 1.2.

In order to prevent the austenite grains to make growth, and consequently to make the pearlite grains fine, required are a certain amount of finely dispersed titanium nitrides or carbo-nitrides. The ratio of the contents of titanium (Ti) and nitrogen (N) is a key to effectuate the aforementioned required conditions. When Ti/N is less than 0.25, the amount of nitrides or carbo-nitrides is not sufficient such that pearlite grains grow coarsely. On the other hand, when Ti/N ratio exceeds 1.2, large size nitrides or carbo-nitrides are generated, which acts as the origins of fatigue failure to reduce the fatigue strength. In addition, in order to meet the aforementioned both requirements regarding the range of titanium content and Ti/N ratio of 0.25 to 1.2 all together, the nitrogen (N) content is required to be controlled in the range of not less than 0.0042 mass % to not more than 0.0480 mass %.

Pb: Not More than 0.03 Mass %.

Since lead (Pb) has become to be regulated to use as described above, the content thereof is desirable to be reduced as low as possible. In adherence to such trend, so called “half leaded steels”, the lead content of which is not less than 0.04 mass % but not more than 0.09 mass %, have been standardized in part of the free cutting steel by the Japanese Automobile Standards Organization, and the effect of such “half lead” on the machinability has been recognized generally. On the other hand, a small content of lead not more than 0.03 mass % can be mixed from scraps or alloying sources in the process of steel melting. The foregoing facts indicate that lead, the content of which exceeds 0.03 mass %, can be considered as added intentionally. In accordance with this, the lead content of the steel according to the present invention is set to not more than 0.03 mass %, and such steel is defined as “non-leaded steel”.

In addition, the steel according to the present invention may contain, besides the requisite alloying elements specified above, alloying elements such as copper (Cu), nickel (Ni), phosphorous (P) and oxygen (O) unless these elements would impair the effects described above. Copper or nickel, the content of which is as high as 0.10 mass %, can be possibly mixed as an inevitable impurity from scraps and the like. Also, phosphorous or oxygen can be mixed as an inevitable impurity during the steel melting process. It is preferable that phosphorous is recommended to be controlled not to exceed 0.0030 mass % because it deteriorates the toughness of steel.

Further, inclusion particles existing in the non heat refined free machining steel are desirable to have a double layered structure from the standpoint of improving the machinability. That is to say, the inclusion particle is formed with a core consisting of oxides of calcium (Ca), magnesium (Mg), silicon (Si) or aluminum (Al), the core of which is covered by a compound of manganese sulfide (MnS) containing calcium sulfide (CaS). In this case, when the oxygen content of the steel is less than 0.0005 mass %, the amount of oxides inside the core of the double layered structure come to be insufficient to improve the machinability effectively. On the other hand, when the oxygen content exceeds 0.01 mass %, the melting point of oxides is lowered and a large amount of free oxides, which do not serve as the core of the double layered structure, are generated. This situation also deteriorates the effect of improvement of the machinability. Therefore, the oxygen content of the steel according to the present invention is desirable to be controlled to not less than 0.0005 mass % but not more than 0.01 mass %.

Still further, when WS represents the sulfur content (mass %) and WO represents the oxygen content (mass %) of the steel described above, the ratio of WS/WO is desirable to be controlled to not less than 8 and not more than 50. When the WS/WO ratio is controlled within the foregoing range, the inclusions having a double layered structure as their main composition, are controlled to have an appropriate morphology to achieve the machinability as superior as that of leaded (Pb) cutting steels according to the prior art. (Note that WS/WO can be expressed simply as S/0 hereinafter.)

The hot forging may be conducted at a temperature not lower than 1,000° C. but not higher than 1,300° C. (naturally at a temperature higher than the A1 transformation temperature). Such temperatures are selected in viewpoint of not only preventing the decarburization but also reducing the deformation resistance to a sufficient level so as to forge the steel into the desired shape with high efficiency. In addition, considering the steel to be applied to an automobile crankshaft having 30 mm to 60 mm of the pin diameter, the forged blank of the crankshaft is supposed to be air-cooled from the forging temperature passing through the A3 transformation point with 30° C./min. to 150° C./min. of cooling velocity. In order to obtain ferrite+pearlite microstructure in the forging after cooling, the critical cooling velocity for creating bainite in the steel must be higher than the above-mentioned upper side of the range of the cooling velocity. Accordingly, the contents of the foregoing requisite alloying elements and the type and the content of the supplemental alloying elements must be selected considering the critical cooling velocity for bainite creation to meet the aforementioned requirements.

The number of sulfides: Not less than 12,000 and not more than 31,000 per 160 mm² of viewing field.

When a steel contains not less than 0.45 mass % of manganese, which serves to generate sulfide, the total volume of sulfides depends on the content of sulfur. Provided that the total volume of sulfide is constant or not changed, the larger the number of sulfide is, the finer the size of sulfides is. The sulfides act as the origins for the stress concentration in the process of cutting so as to enhance the chip breakability. Therefore, as the number of sulfides increases, the number of origins for the stress concentration also increases and results in enhancing the chip breakability. When the number of sulfides is less than 12,000 per 160 mm² of the viewing area, it is insufficient to improve the chip breakability. As a result, elongated and continuous machined chips are generated. These chips likely entangle with the crankshaft work piece or clog the holes in the crankshaft, so that the efficiency in the cutting process comes to be impeded significantly. On the other hand, when the number of sulfides exceeds 310,000, the sulfides become too fine, and result in accelerating the wear of machining tool.

The averaged grain size of pearlite in ferrite+pearlite microstructure: Not less than 14 μm (micro-meters) and not more than 20 μm.

The crankshaft is necessarily going through the straightening process for being removed of the distortion generated during the hot forging and the ferritic nitrocarburizing treatment. When the averaged grain size of pearlite exceeds 20 μm, the straightening property becomes insufficient because of the poor toughness of the steel. On the other hand, when the size of pearlite grains is less than 14 μm, the hardness and the strength become insufficient because of the increase in the creation of ferrite at the increased grain boundaries of fine grains.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 shows a front elevational view of a crankshaft;

FIG. 2 shows optical micrographs comparing the sulfide morphologies in the invention steel example 5 and the comparison steel example 6; and

FIG. 3 shows optical micrographs comparing the averaged grain sizes in the invention steel example 5 and the comparison steel example 6.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

FIG. 1 shows an example of the crankshaft according to the present invention. This crankshaft 1 is configured as such that crank arms 2, which are disposed with a predetermined spacing along a rotational axis O of the crankshaft, are interconnected by means of crank journals 4, a center axis of which is positioned in coincidence with the above mentioned rotational axis O, and crank pins 5, the center axis of which is positioned at a predetermined distance away radially from the above mentioned rotational axis O. Each of the crank pins 5 is formed with a hole 8 for oiling. Each of the crank arm 2 comprises a basal plane portion wherein planes opposing the adjacent crank arms 2 to each other form flat basal planes 2 a. At the projection roots of the crank journal 4 and the crank pin 5 (shaped like a shaft), fillet 7 is formed, the outer diameter of which increases gradually as getting close to the basal plane 2 a. The projection root is shaped concave so that it tends to be stress-concentrated when a bending load is applied on the crankshaft. However, the foregoing fillet 7 serves to lighten the stress concentration on the projection root and to improve the bending strength of the crankshaft as a result.

The crank journal 4 and the crank pin 5 have a shaft-like structure respectively, the cross section of which is a circle. And after the hot forging of the steel having the aforementioned composition, the ferritic nitrocarburizing layer is formed on the whole surfaces of the journals 4 and pins 5. Such a crankshaft 1 is produced as will be described below. To begin with, the raw materials are melted and cast to conform to the compositions of the steel described in detail as above. The billets of the steel are provided for the hot forging, and the forged products are cooled in air from the forging temperature. Although the forgings from the steel having the compositions according to the present invention generate ferrite+pearlite microstructure during air cooling in an atmosphere with a normal pressure, since the forgings employ the above mentioned structure, the total number of sulfides with dimensions observed per 160 mm² of viewing field is more than 1 μm in the forgings will be counted to be in the range of 12,000 to 31,000, and the averaged pearlite grain size thus obtained will be measured to be in the range of 14 μm to 20 μm.

Then the forgings are machined to the configuration of a crankshaft. The cutting work can be performed with significant ease in spite of a low content of lead (Pb) in the steel by virtue both of the excellent machinability because of controlling the total number of sulfides as described before and of the good chip breakability. Subsequent to the cutting work, the forgings are subjected to the ferritic nitrocarburizing treatment in an atmosphere containing ammonia. The deformation or distortion in the forgings generated during the ferritic nitrocarburizing may be removed by way of the cold straightening in a known manner with such as a straightening roll. The steel according to the present invention may keep the averaged pearlite grain size thereof within the range of 14 μm to 20 μm even after the ferritic nitrocarburizing treatment. Accordingly, the forgings may be processed through the straightening after the ferritic nitrocarburizing with ease.

EXAMPLE

Now, results of the experiments carried out for confirming the effects of the present invention will be described below.

To begin with, the raw materials were prepared intending to conform to the chemical compositions indicated in Table 1, melted in an electric furnace and cast into 5-ton ingots. The ingot was rolled by hot rolling down to bars of 88 mm in diameter. Forging stocks taken from the bars were heated to 1,250° C. forged into the configuration of a crankshaft, and then, cooled in air. After cooling, the forgings were subjected to a drilling test by means of a gun drill. As the index, the machinability of the crankshafts of the test steels was evaluated on the basis of the number of drilled holes that were made before the generation of an usual noise or the failure of a drilling tool. Note that the drilling tests were performed with a carbide gun drill of 6 mm in diameter. And the drilling conditions were: drilling speed; 150 m/min., feed; 0.04 mm/rev. and hole depth; 60 mm. TABLE 1 Number of sulfides (number of Chemical composition (mass %) particles/ Classification C Si Mn S Ca Ti S/O Ti/N mm²) Invention 0.39 0.27 0.48 0.058 0.0016 0.0053 25.2 0.35 29394 steel example 1 Invention 0.41 0.26 0.59 0.060 0.0019 0.0052 21.4 0.40 22778 steel example 2 Invention 0.35 0.13 0.40 0.043 0.0007 0.0054 47.8 0.45 16649 steel example 3 Invention 0.40 0.35 0.56 0.045 0.0044 0.0112 11.8 0.70 18439 steel example 4 Invention 0.44 0.38 0.68 0.069 0.0048 0.0090 16.8 0.81 30785 steel example 5 Invention 0.39 0.28 0.55 0.0043 0.0048 0.0066 7.7 0.41 18439 steel example 6 Comparison 0.38 0.22 0.48 0.058 0.0025 0.0057 22.3 0.39 13002 steel example 1 Comparison *0.31 0.29 0.55 0.042 0.0023 0.0085 26.3 0.53 14290 steel example 2 Comparison 0.45 0.16 *0.84 0.061 0.0041 0.0113 13.9 1.03 26827 steel example 3 Comparison 0.44 0.32 0.61 *0.034 0.0009 0.0051 56.7 0.30 *8938 steel example 4 Comparison 0.42 0.22 0.53 0.056 *0.0003 0.0064 22.4 0.46 18342 steel example 5 Comparison 0.38 0.18 0.52 0.040 0.0033 *0.0008 12.9 *0.06 *6438 steel example 6 Comparison 0.39 0.28 0.46 0.054 0.0028 *0.0220 18.6 0.85 21384 steel example 7 Comparison 0.35 0.34 0.58 0.055 0.0018 0.0060 36.7 *0.22 20233 steel example 8 Comparison 0.40 0.26 0.50 0.055 0.0014 0.0051 42.3 0.33 *11526 steel example 9 *Out of the ranges according to the present invention.

Further, subsequent to the hot forging for producing the crankshaft blanks and the machining including the drilling by means of the gun drill on the blanks, the crankshaft of the respective steels were subjected to the ferritic nitrocarburizing treatment in an ammonia atmosphere at 560° C. for 120 min. Thus, the respective steels were completed into crankshafts which may be used practically. The finished crankshafts were subjected to three-point bending tests. These tests were carried out in such manner that while supporting the both ends of the crankshaft with 400 mm of the span and a load is applied to the journal in the center of the crankshaft. The load is increased until a crack is generated in the center journal. The maximum deflection at the initiation of the crack in the center journal is taken as an indicator representing the straightening property of the sample crankshaft.

Still further, the practical crankshafts treated with the ferritic nitrocarburizing were subjected to rotational fatigue tests. The fatigue tests were carried out by varying the maximum applied load, and the maximum applied load under that no fatigue failure generates after 10 million cycles of rotation was defined as the fatigue strength. Table 2 summarizes the results of the tests described above. In addition, metallurgical tests were carried out on a cross-section perpendicular to the center axis of the crank pin with respect to each steel member. After mirror-polishing of the cross-section, the number of sulfides in the viewing field of 160 mm² was counted according to the method described previously. Further, after etching the same cross-section with picric acid, the averaged grain size of pearlite in ferrite+pearlite microstructure was measured according to the method described previously. Table 1 summarizes the results of the total number of sulfides and Table 2 summarizes the results of the average pearlite grain size respectively. TABLE 2 Averaged size Machinability Deflection Fatigue of pearlite (number of Chip by bending strength Classification grains (μm) drilled holes) breakability (mm) (MPa) Invention 17 830 Good 4.52 540 steel example 1 Invention 19 630 Good 4.63 570 steel example 2 Invention 18 900 Good 5.32 510 steel example 3 Invention 15 620 Good 6.21 570 steel example 4 Invention 14 540 Good 6.09 600 steel example 5 Invention 18 350 Good 4.19 540 steel example 6 Comparison *23 610 Good 3.38 540 steel example 1 Comparison 20 950 Good 6.16 450 steel example 2 Comparison 16 240 Good 3.49 570 steel example 3 Comparison 16 200 Poor 4.38 540 steel example 4 Comparison 18 150 Good 4.22 570 steel example 5 Comparison *28 710 Poor 2.74 510 steel example 6 Comparison *22 560 Good 3.66 450 steel example 7 Comparison 17 690 Good 4.51 420 steel example 8 Comparison 20 740 Poor 4.04 510 steel example 9 *Out of the ranges according to the present invention.

The results shown in Table 1 and 2 clearly indicate that with regard to the crankshafts made of the comparison steel examples 1 to 9, wherein any of the chemical composition, the total number of sulfides or the average grain size of pearlite does not conform to the required ranges according to the present invention, any of the machinability, the chip breakability, the bending-straightening property or the fatigue strength does not meet the required levels for the crankshaft. On the contrary, with regard to the crankshafts made of the invention steel examples 1 to 6, wherein all of the chemical composition, the total number of sulfides and the average grain size of pearlite conform to the required ranges according to the present invention, these crankshafts possess satisfactory properties as such that: good machinability, namely, more than 500 of holes can be drilled, and there is no entangling or clogging caused by machined chips; good bending-straightening property, namely, the crankshaft can be deflected for more than 4 mm; and good fatigue strength, namely, the endurance limit is more than 500 MPa. In particular, these effects are remarkable in the invention steel examples 1 to 5, the S/O ratios of which are within the range of 8 to 50. In addition, FIG. 2 shows the optical micrographs comparing the morphologies of sulfide between the invention steel example 5 and the comparison steel example 6 (Magnification of 200). FIG. 3 shows the optical micrographs comparing the grain of the invention steel example 5 and the comparison example steel 6. (Magnification of 400). 

1. A crankshaft made of steel and treated with ferritic nitrocarburizing, the steel containing: C: not less than 0.38 mass % (mass percentage) and not more than 0.42 mass %; Si: not less than 0.15 mass % and not more than 0.35 mass %, Mn: not less than 0.45 mass % and not more than 0.6 mass %, S: not less than 0.04 mass % and not more than 0.06 mass %, Ca: not less than 0.0010 mass % and not more than 0.0050 mass %, Ti: not less than 0.0050 mass % and not more than 0.0120 mass %, N: not less than 0.0042 mass % and not more than 0.0480 mass %, and balanced Fe and inevitable impurities, wherein Ti/N (the ratio of mass percentages of Ti and N) exists in the range of not less than 0.25 but not more than 1.2, wherein microstructure of a cross-section except the ferritic nitrided layer consists of ferrite+pearlite, and the total number of sulfides, the size of which is not less than 1 μm (micro-meters), observed in said ferrite+pearlite microstructure is in the range of not less than 12,000 to not more than 31,000 per 160 mm² of viewing fields, and wherein the averaged grain size of pearlite consisting said ferrite+pearlite is in the range of not less than 14 μm to not more than 20 μm.
 2. The crankshaft according to claim 1, wherein said steel contains Pb (lead), the content of which is not more than 0.03 mass %.
 3. The crankshaft according to claim 1, wherein said steel contains O (oxygen), the content of which is not less than 0.0005 mass % and not more than 0.01 mass %.
 4. The crankshaft according to claim 2, wherein said steel contains O (oxygen), the content of which is not less than 0.0005 mass % and not more than 0.01 mass %.
 5. The crankshaft according to claim 1, wherein WS/WO of said steel is controlled to be not less than 8 and not more than 50, where WS represents the sulfur content in mass percentage and WO represents the oxygen content in mass percentage.
 6. The crankshaft according to claim 2, wherein WS/WO of said steel is controlled to be not less than 8 and not more than 50, where WS represents the sulfur content in mass percentage and WO represents the oxygen content in mass percentage.
 7. The crankshaft according to claim 3, wherein WS/WO of said steel is controlled to be not less than 8 and not more than 50, where WS represents the sulfur content in mass percentage and WO represents the oxygen content in mass percentage.
 8. The crankshaft according to claim 4, wherein WS/WO of said steel is controlled to be not less than 8 and not more than 50, where WS represents the sulfur content in mass percentage and WO represents the oxygen content in mass percentage. 